Toward Engineering Fluorescent nanodiamonds

We have grown nanodiamonds (NDs) with Silicon vacancy (SiV) and Nitrogen vacancy (NV) color centers starting from the organic precursor Adamantane in a diamond anvil cell (DAC) using both resistive and laser heating methods. High resolution transmission electron microscopy (HRTEM) and Scanning Electron Microscopy (SEM) analysis measured a size range of 2 -10 nm, and photoluminescence (PL) analysis found evidence of the Nitrogen and Silicon vacancy color centers. Nanodiamonds with fluorescent color centers have numerous applications like biological markers for drug delivery, and nanoscale sensors of local magnetic fields. For example nanodiamonds containing NVs have been proposed as sensors for a variety of magnetic species in cells, like radicals and protons or phosphorous nuclei, and also as sensors of local electric fields and temperature. However, in order for many of these applications to succeed ultra-small nanodiamonds (USND) with at least one stable fluorescent emitter are needed. Current techniques to grow ultra-small nanodiamonds typically rely on grinding larger crystals or by mechanical shock techniques like detonation of explosives, laser ablation or ultrasound. Unfortunately the resulting nanocrystals tend to have numerous defects especially on the surface. For example, in the case of detonation nanodiamonds the large number of sp2 surface defects leads to agglomeration of the nanodiamonds into ~100 nm clusters. These draw backs hinder the nanodiamonds from being used as biological markers and sensors in environments where the nanodiamond surface purity and size are of utmost importance. Furthermore, the creation of defect color centers inside nanodiamonds is typically probabilistic relying on techniques such as irradiation and annealing of the nanodiamond. These methods tend to place the color-centers near the surface of the nanodiamond resulting in poor fluorescence and coherence properties of the color-centers due to phenomenon such as surface spin traps. Our results not only give nanodiamonds with stable fluorescence and small size, but also imply that it might be possible to grow fluorescent nanodiamond by a high-pressure high-temperature (HPHT) organic seeding process. Such a process is possible if the temperatures and pressures are low enough to allow the creation of diamond without decomposition of the initial organic seed molecule (such as Adamantane). The concept of using an organic seed to grow nanodiamond would also allow the deterministic creation of color-centers at the center of the nanodiamond, giving rise to better fluorescence properties than current probabilistic methods. Such high-quality nanodiamonds would have major impacts in biology, metrology and quantum information sciences. Although our current methods for the nanodiamond synthesis grow small quantities of material (~1 nL) they do allow numerous diagnostics to be employed during growth, so as to allow rapid optimization of growth conditions, and can be readily transitioned to large anvil presses for large-scale manufacturing. Our process therefore may be “scaled-up” to allow the production of large quantities of ultra-small and ultra-pure nanodiamonds for global industrial and biomedical applications.